That the primary pairing state is indeed unconventional is most plainly
demonstrated by the system's strong sensitivity to non-magnetic disorder
[1]}, [2]}.
It is further supported by thermal conductivity measurements [3]}, which report vertical nodal lines, and by the recent quasiparticle
interference measurements of Ref. [4]}.
It is also in line with numerous microscopic theories for unconventional
pairing in this compound [5]}, [6]}, [7]}, [8]}, [4]}, [10]}, [11]}.
Our approach—partially inspired by Ref. [12]}—relies on the existence of a secondary order parameter that is reasonably close in energy and that admixes to the primary phase in the presence
of inhomogeneous strain. However, in distinction to Ref. [12]}, we
neither require an almost perfect
degeneracy of both states for unstrained homogeneous samples, nor
do we expect a second homogeneous mean-field like transition anywhere below the
onset of superconductivity at \(T_{c}\) . Instead, we argue that the
breaking of time-reversal symmetry is exclusively a consequence of strain inhomogeneities in the
material.
